The present invention relates to a process for in situ synthesis of heteropolyacids OR salts thereof on solid supports. More particularly, the present invention relates to a process for the in situ synthesis of acid or cationic form of heteropolyacid of molybdenum or tungsten due to the interaction of the cationic compound with the acidic or salt form of Mo or W and the phosphate ion of a solid support.
Heteropolyacids are prepared by reacting the acid or the ammoniunm/other cationic salts of Mo or W with phosphoric acid or its mono or dibasic ammonium salt. The heteropolyacids are soluble in water or polar oxygenated compounds and are therefore easily deposited on supports such as alumina, silica, zirconia and the like. However, since heteropolyacids possess low thermal stability, they are normally used in their ammonium, potassium, cesium, rubidium, and like element salts. The heteropolyacid in the salt form is also not soluble in the reactant or solvent and is hence not easily washed away during the reaction.
Heteropolyacids and their salts are extensively used as catalysts in several chemical processes such as oxidation, alkylation, isomerisation reactions, since they possess excellent acid-base and redox properties. Heteropolyacids and their salts have been used as catalysts in the place of conventional acids like sulphuric acid, boron trifluoride and aluminum chloride to reduce by-product formation during the reaction and thereby act as environmentally benign catalysts. Often, the acids are used in their acid or their ammonium salt forms.
The acids when used alone or after deposition on a suitable support as catalysts get dissolved in either the reactants or in the solvent and contaminate the product. A problem observed in the case of ammonium slats of heteropolyacids is that uniformity of deposition depends on the pore diameter of the support. Quite often the salt particles are of larger size than the pore diameters of the catalyst and therefore are simply coated on the outer surface of the catalyst as large extrudates.
Deposition of the salts of heteropolyacid on any support is difficult since it is not easily soluble in any solvent. U.S. Pat. No. 5866739 discloses a procedure for the preparation of supported salts of heteropolyacid salts by first impregnating the support with the acid form of the heteropoly compound and then ion exchanging the proton with NH4+, K+, Cs+, Rb+ions. This procedure could lead to partial exchange and therefore ultimately formation of the acid salt. It is also difficult to avoid formation of clusters of the acid or its salt, which reduces the available porosity [Albonetti, et al, J. Catal. Vol 146, (1994) pp.491 -5021].
What is needed is a uniformly distributed ammonium salt of the heteropolyacid on the surface of the support, preferably as a monolayer. The ammonium salt when dispersed uniformly on the support is found to offer higher activity and selectivity in certain reactions.
The main object of the present invention is to provide a process for the synthesis of a heteropolyacid or salt thereof useful as multi-functional catalysts and which is uniformly dispersed on the surface of a solid support.
It is another object of the invention to provide a process for the synthesis of a heteropolyacid or salt thereof resulting in a product offering better selectivity and activity.
It is a further object of the invention to provide an in situ process for the synthesis of a heteropolyacid or salt thereof resulting in a product with enhanced selectivity and activity.
These and other objects of the invention are achieved and the problems associated with the prior art are overcome by the process of the invention described in detail below.
The present invention relates to a novel method of catalyst preparation and its characterisation. The present invention relates to a process for the synthesis of the acid or cationic form of heteropolyacid of Mo or W by the interaction of the cationic compound with the acidic or salt form of Mo or W and the phosphate ion of a solid such as Al, Zr, Ti, or Nb taken as support, wherein the cationic salt, or a mixture of cationic salts of the heteropolyacid is formed in situ on the surface of the support. The phosphate salt of the metal used in step (a) above can be laboratory synthesised or any commercially available phosphate salt. The process of the invention avoids the cumbersome ion exchange process. The process of the invention also avoids the formation of clusters of the salt on the support. The invention also relates to process for the preparation of the catalyst composition comprising:
(a) impregnating the cationic or a mixture of the cationic compounds and the acid or salt form of Mo or W on a phosphate salt of any metal such as Al, Ti, Zr and Nb;
(b) drying in air or inert gas of the product of step (a) above at a temperature in the range of 100-150xc2x0 C.;
(c) careful thermal treatment in air or inert gas at a temperature in the range of 250-400xc2x0 C.
Accordingly, the present invention provides a process for the in situ synthesis of a heteropolyacid catalyst or salt thereof provided on a solid support, said process comprising
(a) impregnating a support comprising the phosphate salt of a metal with a solution of the acid, or ammonium salt or the cationic form or a mixture of cationic forms of molybdenum or tungsten
(b) drying the impregnated support at a temperature in the range of 100-150xc2x0 C.;
(c) subjecting the dried impregnated support obtained at the end of step (b) above to careful thermal treatment at a temperature in the range of 250-400xc2x0 C. to obtain the desired product.
In another embodiment of the invention, the cationic salts are selected from salts of NH4+, K+, Cs+and Rb+salts.
In a further embodiment of the invention, the cationic form of the heteropolyacid comprises the NH4+, K+, Cs+and Rb+salt of molybdenum or tungsten.
In another embodiment of the invention, the heteropolyacid comprises 12-molybdophosphoric acid or 12-tungstophosphoric acid.
In another embodiment of the invention, the support metal comprises Al, Zr, Ti or Nb.